The primary goal of this change is to change `if (pair != *(--m_dict.end()))`
to `if (std::next(iter) != m_dict.end())`. I was experimenting with
changing the underlying type of `m_dict` and found that this was an
issue. Specifically, it assumes that m_dict iterators are bidirectional.
This change should make it so we only need to assume m_dict iterators can move
forward.
Differential Revision: https://reviews.llvm.org/D159150